worktree add: be tolerant of corrupt worktrees

find_worktree() can die() unexpectedly because it uses real_path()
instead of the gentler version. When it's used in 'git worktree add' [1]
and there's a bad worktree, this die() could prevent people from adding
new worktrees.

The "bad" condition to trigger this is when a parent of the worktree's
location is deleted. Then real_path() will complain.

Use the other version so that bad worktrees won't affect 'worktree
add'. The bad ones will eventually be pruned, we just have to tolerate
them for a bit.

[1] added in cb56f55c16 (worktree: disallow adding same path multiple
    times, 2018-08-28), or since v2.20.0. Though the real bug in
    find_worktree() is much older.

Reported-by: Shaheed Haque <shaheedhaque@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2025-worktree-add.sh

@@ -570,4 +570,16 @@ test_expect_success '"add" an existing locked but missing worktree' '
 	git worktree add --force --force --detach gnoo
 '
 
+test_expect_success '"add" should not fail because of another bad worktree' '
+	git init add-fail &&
+	(
+		cd add-fail &&
+		test_commit first &&
+		mkdir sub &&
+		git worktree add sub/to-be-deleted &&
+		rm -rf sub &&
+		git worktree add second
+	)
+'
+
 test_done

worktree.c

@@ -222,9 +222,12 @@ struct worktree *find_worktree(struct worktree **list,
 		free(to_free);
 		return NULL;
 	}
-	for (; *list; list++)
+	for (; *list; list++) {
-		if (!fspathcmp(path, real_path((*list)->path)))
+		const char *wt_path = real_path_if_valid((*list)->path);
+
+		if (wt_path && !fspathcmp(path, wt_path))
 			break;
+	}
 	free(path);
 	free(to_free);
 	return *list;